Fundamental changes in expression of xenobiotic processing genes (XPGs) such as those that encode the Phase 1, 2, and 3 proteins occur during the progression from the neonatal period to the adult. Progress has been made in understanding these changes, as the regulation of these genes has been linked in part through expression of the family of xenobiotic nuclear receptors (XNRs), which partner in relationship to the retinoid X- receptor. But explanations for these differences and the mechanisms that regulate expression of the XNRs have not been elucidated. Experiments conducted with humanized UGT1 (hUGT1) mice that express all nine of the UGT1A genes have revealed that neonatal hUGT1 mice develop dramatic hyperbilirubinemia during the early neonatal period. Two key factors result in this syndrome. First, the human UGT1A1 gene is repressed in liver and is transcriptionally blocked by the repressive actions of PXR. Second, developmental expression of intestinal UGT1A1 late in the neonatal period leads to a reduction of hyperbilirubinemia and clearance of serum bilirubin. These findings suggested that the UGT1A1 gene was actively repressed during neonatal development, and led us to examine the role of the transcriptional co-repressor protein NCoR1. In these experiments, we selectively targeted the deletion of the NCoR1 gene in either the liver (hUGT1/NCoR1?Hep mice) or intestines (hUGT1/NCoR1??HepIEC mice) of hUGT1 mice. Deletion of NCoR1 in liver tissue had no effect on serum bilirubin levels, but deletion of the gene in intestinal tissue completely reversed neonatal hyperbilirubinemia. Consistent with the reduction of serum bilirubin in hUGT1/NCoR1??HepIEC mice was the dramatic induction of intestinal UGT1A1 gene expression. This intriguing finding led us to examine in greater detail the role of NCoR1 in early neonatal and adult expression of the XPGs in intestinal tissue. Our preliminary findings clearly indicate that the deletion of the c-repressor NCoR1 leads to induction and regulation of a series of XPGs in neonates that are different from those induced in adult intestines. Since NCoR1 represses gene transcription by forming a co-repressor complex with selective XNRs, we hypothesize that NCoR1 is controlling the repression of XPGs in a developmental fashion by forming complexes with different XNRs. To examine this hypothesis, we are going to characterize the expression patterns of the XPGs and the epigenetic events that control expression of these genes as a result of NCoR1 de-repression. These findings will allow us to identify the nuclear receptor involved in NCoR1 repression by associating gene expression patterns to known XNR function. In addition, this work will reveal the underlying mechanisms linking repression of XPG expression during neonatal development and the regulatory events that promote differential induction of these genes in adult mice. It is hopeful that this work will reveal for the first time the molecular evens that dictate developmental expression of the XPGs. These findings will be important in determining the link between toxicant exposure, metabolism and a toxic episode that is favored during neonatal development.